1. Field of the Invention
This invention relates to a method for maintaining the electrochromic activity of an electrochromic material. The electrochromic material may be either an anodic electrochromic material or a cathodic electrochromic material.
2. Discussion of the Related Art
Reference is made copending U.S. patent application Ser. No. 137,633, filed Dec. 24, 1987, to Demiryont for "NEW ELECTROCHROMIC MATERIAL, METHOD OF MAKING, AND NEW ELECTROCHROMIC DEVICE" for a complete discussion of electrochromic activity of both the anodic and the cathodic variety. That application is hereby incorporated by reference for such teachings.
Most simply, an electrochromic device comprises an electrochromic material layer in contact with an ion conductor material layer which is a source of ions. The electrochromic material is an insulator (dielectric material). These two layers are positioned between electrodes, at least one of which is transparent. Because the electrochromic material is a dielectric material, when a voltage is applied across the electrodes, an electric field is generated within the electrochromic material. This electric field can cause chemical changes and corresponding color changes in the electrochromic material. The electrochromic material may change color, e.g., from colorless to blue, when it "switches" from one electrochromic state to another. By reversing the polarity of the applied voltage and hence the electric field, the electrochromic material can be switched back, e.g., from blue to colorless, that is, from its "colored" to its "bleached" state. Electrochromic devices are well known in the art and have been disclosed for example, in U.S. Pat. Nos. 3,521,941, 4,170,406, 4,194,812, 4,344,674, 4,505,538 and 4,645,308.
Optimally, the electrochromic material has good electrochromic activitY and maintains this activity during a substantially large number of switches (cycles) of the electrochromic material. Such electrochromic activity includes the ability of the electrochromic material to be repeatedly colored during different cycles to the same intensity of color, should that be desired. Still further, the material should maintain the ability to switch rapidly in the presence of an electric field of reversing polarity, and at the same magnitude of positive or negative applied voltage, even after a large number of switches.
The electrochromic activity of an electrochromic material not subjected to the method of this invention tends to decrease when it is repeatedly switched between its various electrochromic states. For example, the intensity of the color which can be brought about in the electrochromic material is generally greatly decreased as the material is switched. In such cases, in an attempt to bring about the same intensity of color as in the previous cycles, the magnitude of the applied voltage is increased. This generally only provides a temporary correction of the situation. In time, even increasing the magnitude of the applied voltage generally does not provide the previous intensity of color. Another problem with electrochromic materials not subjected to this invention is that switching generally takes longer with each additional cycle.
Good cathodic electrochromic materials should be of the general formula: MO.sub.x, where M is a variable valence metal and x is a number less than that which will provide a stoichiometric compound. That is, for good cathodic electrochromic materials the compound should be oxygen deficient, i.e., in a reduced state. Thus, as described in copending application U.S. Ser. No. 138,234 to Demiryont filed Dec. 28, 1987, WO.sub.x, with x less than 3 is a good cathodic electrochromic material while stoichiometric tungsten oxide, WO.sub.3, is not. (Presently WO.sub.3 is understood to be electrochromic but of less than desirable electrochromic quality.) Similarly, as described in copending application U.S. Ser. No. 179,825 to Demiryont filed Apr. 11, 1988, VO.sub.x with x less than 2.5 is a good cathodic electrochromic material, while stoichiometric V.sub.2 O.sub.5 is not an electrochromic material. In order for such an oxygen deficient, cathodic electrochromic material to maintain its electrochromic activity during switching, it needs to be maintained in its oxygen deficient (reduced) state.
In the presence of oxygen or oxygen providing materials (as may be present in an ion conductive layer), the oxygen deficient electrochromic material may undesirably oxidize to its stoichiometric form. When this happens, the electrochromic activity deminishes. Some cathodic electrochromic materials such as reduced lead oxide and reduced bismuth oxide are highly unstable in their reduced state. For these materials, oxidation may begin as soon as the electrochromic material is subjected to an environment comprising oxygen or oxygen providing materials (e.g., O.sub.2 or H.sub.2 O), even absent an applied voltage. Other cathodic electrochromic materials, such as reduced tungsten oxide, are generally stable in their reduced state in the presence of an environment comprising oxygen or oxygen providing materials, absent application of a voltage of positive polarity to the material. Oxidation appears to begin only after this kind of electrochromic material is subjected to an oxidizing environment and to an application of a voltage of positive polarity, as occurs during switching of an electrochromic material. As disclosed above, oxidation of reduced tungsten oxide brings about a corresponding decrease in the electrochromic activity of the tungsten oxide.
Likewise, it is believed that subjecting a fully oxidized anodic electrochromic material to a reducing environment brings about reduction of the electrochromic material with a corresponding decrease in its electrochromic activity.
It thus would be desirable to provide a method for maintaining the electrochromic activity of an electrochromic material at least during operation of the device and, if necessary, even before the device is subjected to operation.
This specification teaches a method for maintaining the electrochromic activity of an electrochromic material over a prolonged period of time during which it is subjected to repeated switching (cycling). The electrochromic activity may be maintained for either an anodic electrochromic material or a cathodic electrochromic material.
A search conducted on the subject matter of this specification resulted in the citation of U.S. Pat. Nos. 4,350,414; 4,435,048; and 4,645,308. Also cited was an article from The Journal of Applied Physics, 53(1), January 1982, entitled "Prussian-Blue-Modified Electrodes: An Application For A Stable Electrochromic Display Device". None of the cited material teaches or suggests a method such as to be disclosed herein for maintaining the electrochromic activity of an electrochromic material. However, the material cited will be discussed individually below.
U.S. Pat. No. 4,350,414 issued on Sept. 21, 1982 for an "All Solid-State Electrochromic Device". The all solid-state electrochromic device disclosed features a structure composed of an oxidizable film capable of a redox reaction which shows a change in the transmittance in a certain wavelength range in the oxidized state. A reducible film is also provided which is capable of a redox reaction which shows a change the transmittance in a certain wavelength range in the reduced state. An insulating film is provided between the aforementioned films. The insulating film allows proton conduction but prohibits electron conduction. A pair of electrodes are provided between which the mentioned three films are positioned.
U.S. Pat. No. 4,435,048 issued on Mar. 6, 1984, for an "Electro-Optical Device and Electro-Optical Light Controlling Device." The patent discloses an electro-optical device comprising an electrolyte layer held between a pair of electrodes and an electrochromic material placed on at least one of the electrodes. The electrolyte layer comprises an organic material and a chelating agent and an electro-optical light controlling device comprising an electrolyte layer held between a pair of transmissive electrodes and an electrochromic material placed on at least one of the electrodes. The electrolyte layer comprises a nonliquid material of an adhesive or tacky high polymer and a material having coordinating function to a metal.
U.S. Pat. No. 4,645,308 issued on Feb. 24, 1987 for a "Low Voltage Solid-State Lateral Coloration Electrochromic Device". The patent discloses a solid-state transition metal oxide device comprising a plurality of layers having a predisposed orientation including an electrochromic oxide layer. Conductive material including anode and cathode contacts are secured to the device. Coloration is actuated within the electrochromic oxide layer after the application of a predetermined potential between the contacts. The coloration action is adapted to sweep or dynamically extend the length of the electrochromic oxide layer.
The Journal of Applied Physics article described an electrochromic display device based on a prussian-blue-modified electrode. Prussian blues are deposited electrochemically in a solution of ferric-ferricyanide. Current flow at +0.2 and +1.0V is due to a reduction of ferric ions and the oxidation of ferrous ions in the prussian-blue coating respectively.